Temperature effects creep

Table 1 is condensed from Handbook 44. It Hsts the number of divisions allowed for each class, eg, a Class III scale must have between 100 and 1,200 divisions. Also, for each class it Hsts the acceptance tolerances appHcable to test load ranges expressed in divisions (d) for example, for test loads from 0 to 5,000 d, a Class II scale has an acceptance tolerance of 0.5 d. The least ambiguous way to specify the accuracy for an industrial or retail scale is to specify an accuracy class and the number of divisions, eg. Class III, 5,000 divisions. It must be noted that this is not the same as 1 part in 5,000, which is another method commonly used to specify accuracy eg, a Class III 5,000 d scale is allowed a tolerance which varies from 0.5 d at zero to 2.5 d at 5,000 divisions. CaHbration curves are typically plotted as in Figure 12, which shows a typical 5,000-division Class III scale. The error tunnel (stepped lines, top and bottom) is defined by the acceptance tolerances Hsted in Table 1. The three caHbration curves belong to the same scale tested at three different temperatures. Performance must remain within the error tunnel under the combined effect of nonlinearity, hysteresis, and temperature effect on span. Other specifications, including those for temperature effect on zero, nonrepeatabiHty, shift error, and creep may be found in Handbook 44 (5). The acceptance tolerances in Table 1 apply to new or reconditioned equipment tested within 30 days of being put into service. After that, maintenance tolerances apply they ate twice the values Hsted in Table 1.

Different viscoelastic materials may have considerably different creep behavior at the same temperature. A given viscoelastic material may have considerably different creep behavior at different temperatures. Viscoelastic creep data are necessary and extremely important in designing products that must bear long-term loads. It is inappropriate to use an instantaneous (short load) modulus of elasticity to design such structures because they do not reflect the effects of creep. Viscoelastic creep modulus, on the other hand, allows one to estimate the total material strain that will result from a given applied stress acting for a given time at the anticipated use temperature of the structure. 

Creep rates of three glassy polymers are much greater during electron irradiation than before or after. Radiation heating is eliminated as a possible cause. Essentially the same concentration of unpaired electrons and ratio of cross-linking to scission were found in polystyrene samples in the presence or absence of stress. The effects of radiation intensity, stress, and temperature on creep during irradiation are examined. The accelerated creep under stress is directly related to a radiation-induced expansion in the absence of stress. This radiation expansion is decreased by increase in temperature or plasticizer content and decrease in sample thickness. It is concluded that gas accumulation within the sample during irradiation causes both the expansion under no stress and the acceleration of creep under stress. 

Figure 10.30 The effect of temperature on creep curves. When the stresses T2> Ti. When the temperatures T, T2 and To, are equal, the stress values ai, (J2 and 0-3 lie in the sequence...

Yun, H.M., J.C. Goldsby, and J.A. DiCarlo. 1995c. Time-temperature effects on the rupture and creep strength of oxide fibers, to be published. Yun, H.M., and J.A. DiCarlo. 1996. Time/temperature dependent tensile strength of SiC and Al203-based fibers. Pp. 17-25 in Advances in Ceramic-Matrix Composites III, Vol. 74 in Ceramic Transactions, N.P. Bansal and J.P. Singh (eds.). Westerville, Ohio American Ceramic Society. 

The first two terms on the right-hand side of equation [12.6] are viscoelastic terms proposed by Schapery, where e represents uniaxial kinematic (or total) strain at time t, o is the Cauchy stress at time t, is the instantaneous compliance and AD(r[i ) is a transient creep compliance function. The factor g defines stress and temperature effects on the instantaneous elastic compliance and is a measure of state dependent reduction (or increase) in stiffness. Transient compliance factor gi has a similar meaning, operating on the creep compliance component. The factor gj accounts for the influence of loading rate on creep. The function i ) represents a reduced timescale parameter defined by ... 

Rate of degradation promoted by elevated temperature also creep effects. May aid postcuring, and may plasticise and toughen cured adhesive. 

Examples of creep effects covering environment, fibre type, resin type, effects of temperature, effects of strain and effects of overload are given in a variety of references at the end of this section. Creep effects are also affected by cyclic and intermittent loading. At cyclic frequencies above a few hertz the effects of heating of the material from hysteresis can be a factor in increasing creep. 

Effect of temperature on the creep of NT154 (a) Strain as a function of exposure time (b) Minimum creep rate as a function of temperature. Transient creep effects are evident from the time required for the data to approach a common Arrhenius curve. Approximately 1500 hr of creep was required to approach steady state creep. Stress 100 MPa. 

ACI 209R-92 (reapproved 2008) Prediction of Creep, Shrinkage and Temperature Effects in Concrete Structures. 

ACI (1999) Prediction of creep, shrinkage and temperature effects in concrete structures, ACI Manual of Concrete Practice, Com. 209, Part 1. 

Moreover, it is relevant to note that the creep behaviour of the ramming paste in this case study is fxmction of the baking index only, even at various temp>eratures. In fact, up to now, no creep/relaxation results are available for the ramming p>aste at various elevated temperatures. Also, it should be mentioned that the baking and the temperature effects are considered independent. The baking effect is related to the microstructure of the p>aste which is a fxmction of the highest temperature reached by the paste (irreversible process), while the temperature effect is related to the actual temperature of the peste (reversible process). 

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